β-Carbolines. 2. Rate Constants of Proton Transfer from Multiexponential Decays in the Lowest Singlet Excited State of Harmine in Water As a Function of pH

Abstract

The -carbolines present a complex problem involving multiple equilibria in the excited state in hydrogenbonding solvents including water. Three excited state species exist: neutral, cation, and zwitterion. Here we examine the multiple equilibria and excited state kinetics of harmine, using time-resolved and steady state fluorescence techniques. From an analysis of the multiexponential decays, measured at the emission wavelengths of the three species as a function of the pH, seven unknowns (four rate constants and three reciprocal lifetimes) were determined. Data analysis was made both by a previously reported numerical method and by analytical solution of the differential equation set. The results obtained accurately describe the independently obtained steady-state fluorescence results. The dramatic modifications of the equilibria and rate constants between the ground and excited states can be understood on the basis of the significative changes in charge densities on the two nitrogen atoms of harmine upon excitation. Mechanisms are proposed for the formation of excited state cation and zwitterion beginning with the excited state neutral molecule.

Ground‐ and Excited‐State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives

“…order of magnitude to those reported for the ICT states of other compounds, such as 9-aminoacridinium derivatives and N-phenylpyrrole [68,69]. Interestingly, t 0 ICT is also in excellent agreement with the values reported by different authors for the decay time of the zwitterionic phototautomer of betacarboline [22,28,50].…”

“…Furthermore, as mentioned, the lifetime of the ICT state, 4.0 ns, coincides with the values published by different authors for the lifetime of the presumed zwitterionic phototautomer of BC[22,28,50]. Thus, the origin of the large Stokes-shifted fluorescence band of betacarbolines, around 520 nm, is not due to UV-vis absorption spectra of the BC-HFIP (8.6 Â 10 À3 À8.6 Â 10 À2 M) system in cyclohexane.…”

“…Among the great variety of substrates possessing potential hydrogen bond donor/acceptor sites, the betacarboline ring, 9H-pyrido [3,4-b]indole (BC) [20] (Scheme 19.1), has been widely studied [21][22][23][24][25][26][27][28][29][30][31][32][33]. These rings are the structural units of numerous naturally occurring alkaloids that possess a wide range of biological and pharmacological properties [34][35][36][37][38][39][40][41][42][43][44][45].…”

Solid-phase fluorescence and ionization efficiency in negative-ion matrix-assisted laser desorption/ionization of neutral oligosaccharides: Interaction between β-carboline matrix and ammonium salt

“…The peaks occurring at 365 and 407 nm came from the harmine hydrochloride in methanol solution (Figure 3a). The fluorescence at 407 nm originated from protonated harmine below pH 7.0 in aqueous solution [28]. Here, the molar ratios of added ammonium chloride to harmine were extremely high in their liquid-phase fluorescence spectra (Figure 3b), and did not correspond to increases in the ion abundance ratio of the chloride-anionized molecules (Figure 2).…”

Solution phase photophysics of 5,7-dimethoxy-2,3,4,9-tetrahydro-1H-carbazol-1-one: Analysing the lineaments of a new fluorosensor to probe different micro-environments

“…Indole skeletons are also present in bioactive molecules like carbazoles, carbolines etc. [6][7][8][9][10]. This skeleton appears to be a potential foundation for de novo design and synthesis of derivatives capable of protecting the nervous system and for many other biological activities [11].…”